The fundamental understanding of electron transport in individual molecules wired between two metal electrodes is one of the key focuses in molecular electronics due to its potential applications in molecular computers and future bio-nanoelectronics. Due to the small sizes of single molecules, it has been difficult to obtain single molecule conductivities. Only recently, several critical technical approaches for forming stable metal-molecule-metal junctions have become possible and the conductivity of a series of molecules has been reported. These approaches are, for example, conducting probe atomic force microscopy (CP-AFM), nanopores, mercury-drop junctions, electromigrated break junctions, mechanically controllable break junctions, and scanning tunneling microscopy based break junction (STMBJ). Among them, the STMBJ was the latest one reported but has been widely used ever since its invention.
The STMBJ technique has several advantages. First, the target molecule is not required to be inserted in a less conductive matrix. For example, CP-AFM requires the target molecule to be inserted into a less conductive matrix so that one end of the molecule can be chemically bonded to the conductive substrate and the other end is facing up to provide a bonding site to the second electrode, i.e., the scanning probe microscope tip. This is often difficult for molecules that have complicated geometries or those that do not have a suitable less conductive matrix environment. STMBJ does not have this limitation. The molecules for STMBJ measurement can be protruding up from the substrates in a condensed monolayer, in clusters, or a single molecule scattering on the substrate surfaces. Second, STMBJ can be conducted in several environments. The STMBJ measurement can be carried out in vacuum, air, insulating liquid and conductive liquid environment. Third, the molecular conductivity generated by STMBJ is the closest one to theoretical approaches and is more convincing. That is because the conductivity is measured only when the molecules are bridged between the electrodes and stretched in full length. In STMBJ, bridged molecules are separated from other non-bridged molecules and the two electrodes are separated far enough so that direct tunneling current is negligible. Therefore, the measured current is mostly from the through-molecule current that reflects the real molecule conductivity. Finally, owning to the STMBJ's simple geometry that is only composed of target molecule and metal electrodes, it is easier for theorists to build models to perform theory studies.
STMBJ measures the conductivity mainly in the stretching process as a function of time elapsed or stretching distance. The terraces or plateaus in the conductivity-versus-stretch distance curves are attributed to current through bridged molecules. The conductance histogram from such terraces shows a series of peaks appearing at the integer multiples of a fundamental value that was used to identify single molecule conductivity.
In the initial studies, there was only one set of histogram peaks reported. However, recent approaches have revealed that there are multi-set peaks. Each set can deduce a single molecule conductivity that has a variation of 1˜10. This significant difference raises a fundamental issue of molecule junctions, the contact/geometry effects, which have been well accepted to account for the observed multi-set peaks. The role of the electrode-molecule contacts and the specific geometry in molecular junctions has been the least controllable aspects of the experiments and proper methods for a detailed study and further investigations are highly needed.
In traditional STMBJ technique, the conductivity is measured during a continuous stretching of the molecular junction, which produces continuous modification on the contact configurations and subsequently could complicate the conductivity measurements. Therefore, it is still a big challenge to separate the influences of the stretching movement of the electrode from real conductivity. From the point of retrieving real molecule conductivity, an ideal measurement should be carried out under static or quasistatic electrode configurations to avoid motion caused contact or changes in geometry. Introducing additional motion into the stretching and/or engaging process can be an effective way to modulate or disturb the junction geometry/contact so that more details can be obtained about the molecule junction system's stability and other information.
Modulation can also be introduced to the sample bias applied across the electrodes to provide a desired electric field that is very useful to study substances with dielectric responses, for example, nano-particles and biological samples. This can yield numerous new applications.
Finally, simultaneous measurement of conductance and force can give information on both electronic and mechanical properties of both the junction system and the measured substance. It can shed more light onto the contact/geometry effects. A similar existing technique is the current sensing atomic force microscope (CS-AFM) based breakjunction (CSAFM-BJ). Although this technique measures both force and conductivity during the stretching process, it uses only the deflection/force as feedback to control the engaging process. Compared to current-feedback, the disadvantage of deflection/force feedback is lack of contact sensitivity especially when AFM tips with a high spring constant are used. By using current as feedback, the sensitivity can be greatly enhanced due to the exponential dependence of the tunneling current over electrode-sample separation.
A major reason that current CSAFM-BJ is not using current as feedback is due to a frequent exception that the current may disappear during normal measurements. Once that happens, the system looses feedback control and in most cases will result to CSAFM tip crashes.
Thus, there is a need for a highly integrated system as well as an effective method to systematically investigate the electronic and mechanical properties of molecule junctions as well as that of the molecule itself.
It is an object of the invention to provide systems and methods for measuring properties of junctions in nanodevices.